This application claims a priority based on Japanese Patent Application No. 2000-295425 filed on Sep. 25, 2000, the entire contents of which are incorporated herein by reference for all purposes.
The present invention relates to a thin film semiconductor element for a liquid crystal display, particularly to a thin film transistor element for a liquid crystal display using crystalline silicon by a low temperature processing and a method of manufacturing the same.
In recent years, a liquid crystal display panel using a thin film transistor element of amorphous silicon is widely used as a display device in a personal computer, an information terminal equipment and the like.
However, it has been desired to develop a thin film transistor element capable of a high speed driving in accordance with an improved fineness of a display device and a rapid processing of an increasing amount of information.
Such a thin film transistor element is described in, for example, (1) xe2x80x9c""99 Updated Liquid Crystal Process Technology, published by Nikkei Business Publications Inc., 1999, page 54xe2x80x9d. It is described that an amorphous silicon film is formed on a glass substrate by a PE-CVD method (Plasma Enhancement-Chemical Vapor Deposition Method), followed by performing a dehydrogenation annealing processing on the amorphous silicon film. Then, after boron atoms are added to a channel region, an excimer laser annealing processing is performed so as to polycrystallize the amorphous silicon film, thereby forming a thin film transistor element or crystalline silicon.
It is also reported in, for example, (2) Japanese Patent Laid-Open No. Hei 11(1999)-354801 gazette that an amorphous silicon film is immersed with a solution containing ozone so as to form an oxide film on the surface of the amorphous silicon film, followed by removing the oxide film by using a hydrofluoric acid solution and subsequently performing a laser annealing so as to crystallize the amorphous silicon film.
It is also reported in, for example, (3) Japanese Patent Laid-Open No. Hei 11(1999)-16866 gazette that, after a native oxide film of an amorphous silicon film is removed by a wet etching, an oxide film is formed on the surface of the amorphous silicon film by immersing the amorphous silicon film with an aqueous ozone solution or an aqueous hydrogen peroxide solution, followed by performing a laser annealing so as to crystallize the amorphous silicon film.
Further, a semiconductor device and a method of manufacturing the same are described in, for example, (4) Japanese Patent Laid-Open No. Hei 10(1998)-64819 gazette. It is described that, in forming an amorphous silicon film on a substrate, a metal for selectively fostering the crystallization is added so as to promote the crystal growth in a predetermined crystal growing direction, thereby forming a crystalline silicon film.
It should be noted in the thin film transistor element using the conventional crystalline silicon film described above that, in general, a silicon film is a polycrystalline silicon (polysilicon) film. In the polysilicon film, there is a close relationship between the crystal grain size (diameter) and the electron mobility. Specifically, the electron mobility is decreased with decrease in the crystal grain size. One of the causes of the particular relationship is considered to reside in that carriers migrating within the polysilicon film, i.e., electrons, are scattered in the grain boundary present in the polysilicon film.
It is generally said that, in the case where the polysilicon film is formed by performing the laser annealing processing to an amorphous silicon film, a sufficient crystal growth cannot be achieved when an irradiation energy density of a laser beam is low, with the result that the crystal grains of the formed polysilicon have a size not larger than 100 nm in many cases.
In the case of employing prior art (1) described above, it is possible to form crystal grains having a grain size not smaller than 100 nm by increasing the energy density of the laser beam used for irradiating the amorphous silicon film. In this case, however, the nonuniformity in the crystal grain sizes is Increased with increase of the crystal grain size, causing a problem that nonuniformity in the electric characteristics is generated in the formed polysilicon film.
It should also be noted that, where a polysilicon film is grown by employing a laser annealing method, a serious problem is generated in the manufacturing process that abnormal protrusions not smaller than 50 nm from the crystal grain boundary are generated.
Moreover, in the above described prior art (2), it is possible to prevent the protrusion of the crystal relating to the crystal grain boundary and derived from the laser annealing processing by a performing the laser annealing processing after the oxide film is removed by using the aqueous hydrofluoric acid solution. In an ordinary case, it is necessary to carry out the cleaning processing with, for example, pure water or the like after the processing with the aqueous hydrofluoric acid solution.
However, since the amorphous silicon surface after removal of the surface oxide film exhibits the hydrophobic, fine water droplets not larger than several micrometers remain on the surface of the: amorphous silicon after the cleaning processing. And then, these water droplets cause silicon atoms to elute out of the surface of the amorphous silicon so as to form protrusions not smaller than at least 50 nm.
Since these protrusions remain as they are even after the laser annealing processing to form polysilicon, these protrusions cause a defocusing in a subsequent photolithography step, with the result that the reliability of the insulating film laminated on the polysilicon film is impaired due to an insulation defective or the like.
Prior art (3) described above shows that an oxide film having a thickness of 1 to 3 nm is formed on the amorphous silicon film by using an aqueous ozone water or an aqueous hydrogen peroxide water, followed by drying the oxide film and subsequently performing the laser annealing processing to the dried oxide film. However, since the oxide film formed on the surface has a large thickness, a serious problem has been generated that abnormal protrusions not smaller than 50 nm from the crystal grain boundary were generated even if the laser annealing was directly performed to the amorphous silicon to form a polysilicon film.
Furthermore, in prior art (4) described above, it is possible to form the polysilicon film of a (111) plane orientation exhibiting high mobility characteristics by the addition of the metal to foster the crystallization. However, in the method described in this prior art, an additional step of adding the metal is required so as to make the manufacturing process more complex. In addition, in the crystal growing process carried out under a low temperature, i.e., not higher than 450xc2x0 C., the crystal growing rate becomes markedly low in general, leading to a low productivity. This is a serious problem in view of the social demands for realization of a liquid crystal display using the polysilicon thin film transistor.
The contents given below are not described in the aforementioned prior arts.
An object of the present invention is to provide a polysilicon film for a liquid crystal display, which has a low nonuniformity of a crystal size, suppresses occurrence of the protrusion on the surface, exhibits a preferred (111) plane orientation, and has a high electron motility.
The purpose of the present invention can be achieved by preparing a thin film consisting of an aggregate of crystal grains preferentially oriented on the (111) plane that is parallel to the substrate surface, and by forming a thin film Si, Ge or SiGe semiconductor element, in which the crystal grains have an average crystal grain size not larger than 300 nm on the surface of the thin film.
It is possible to utilize, for example, an X-ray diffraction intensity ratio of the crystal surface as an index of the preferred orientation. The purpose of the present invention can be achieved by setting the X-ray diffraction intensity ratio I(111)/I(220) in the (111) plane and the (220) plane, when measuring the X-ray diffraction on the plane parallel to the substrate surface, at 30 or more.
Also, it is possible to suppress the variation in the characteristics of the crystal grains so as to obtain a stable crystal grain boundary by setting the standard deviation of the grain diameters of the crystal grains at 30% or less of the average grain size.
Also, it is possible to form a polysilicon thin film, in which the formation of protrusions at the crystal grain boundary is suppressed, by setting the standard deviation of the surface roughness at 10% or less of the average grain size.
Further, it is possible to obtain a polysilicon thin film low in protrusion occurrence in the crystal grain boundary by forming an aggregate of columnar crystals oriented on the approximate (111) plane in a direction parallel to the substrate surface.
A polysilicon thin film transistor having a high electron mobility and applied for the liquid crystal display can be achieved by setting the average electron mobility of the polysilicon thin film of the present invention described above at 200 cm2/v""s or more, by constituting the thin film described above with an aggregate of crystal grains preferentially oriented on the (111) plane in a direction parallel to the substrate surface, and by setting the average crystal grain diameter of the crystal grains on the surface of the thin film at 2% or more of a channel length of the transistor.
It is possible to form a crystalline semiconductor thin film applied for the liquid crystal display, in which the protrusion occurrence on the surface can be suppressed in spite of the high crystallinity, by forming an amorphous semiconductor thin film on the substrate, by controlling the thickness of the oxide film formed on the thin film to fall within a range not less than 0.1 nm and not more than 0.4 nm, and further by annealing the amorphous thin film by a laser beam irradiation so as to crystallize partially or entirely the amorphous thin film,
The thickness of the oxide film can be controlled to fall within a range of 0.1 nm and 0.4 nm by immersing the amorphous semiconductor thin film in a solution containing HF and, further, in an aqueous solution containing H2O2.
Further, the purpose of the present invention can be achieved by removing the oxide film formed on the surface of the amorphous semiconductor thin film by immersing the amorphous semiconductor thin film in the aqueous solution containing at least HF, and further by irradiating the semiconductor thin film with a ultraviolet (UV) light under an ambient containing oxygen or by immersing the semiconductor thin film in the aqueous ozone solution.